1. Field of the Invention
The invention relates to the processing and preferably to the recording and/or playback of color video signals within a limited bandwidth of not more than 3 mHz without a separate channel for the color information. More particularly, the invention relates to a method and circuits whereby National Television Systems Committee (NTSC), Phase Alternate Lines (PAL), or other standard broadcast formats are converted into an encoded signal for recording upon a suitable medium, specifically magnetic tape. Upon playback, the required color video broadcast format is reconstituted from the recorded encoded signal. Although the invention relates to the recording and/or playback of color video signals, it is equally applicable to the processing within a limited bandwidth of any modulated broadband signal which also utilizes a modulated subcarrier.
2. Description of Prior Art
It is well known that various colors can be produced by the combination of the three primary colors red, green, and blue in the proper relationship. Color video information, or chrominance information, is broadcast by imposing upon a black and white television signal a modulated subcarrier containing the color difference signals. The NTSC and PAL-M systems use a 3.58 mHz subcarrier for color transmission, while PAL-B uses a 4.43 mHz subcarrier. All these systems employ quadrature modulated color difference signals.
In video recording, there is always present the problem of how to record the color subcarriers. These subcarriers are high frequency; they often have large amplitudes; and their recording and subsequent playback must not introduce phase errors, which result in color changes. Recording of color subcarriers is especially a problem in home video recorders since they are typically designed with limited bandwidth to reduce production costs. Many such video recorders remove the subcarrier with its chrominance information, heterodyne it down to 500 kHz, and record this signal on tape. Two recording channels are used; one is the heterodyned color information, and the other is the luminance signal or information. In such a system, the color information is recorded as an amplitude modulated signal without frequency modulation.
Prior art systems of the kind just mentioned, which use heterodyning-down of the color subcarrier, are deficient from the standpoint of color lock stability and bandwidth on playback--keeping in mind that the recovered color subcarrier must represent the original signal very accurately as to frequency and phase so that proper demodulation in the TV receiver is insured.
Line sequential color television systems are known wherein the NTSC color difference signals are demodulated according to the phase angle of their subcarriers in a predetermined relationship to the phase angle of the color burst synchronization signal included in the video signal transmitted by the TV station. In this regard, see, for example, German Published Patent Application DAS 1,256,686 and U.S. Pat. No. 3,560,635. For the demodulation process of bi- or trisequential signals the use of two or three demodulators is necessary. After demodulation, the color difference signals are filtered by means of two or more filters and, by means of a sequential switch, are converted into line sequential color difference signals. Thereafter, these color difference signals are added to the luminance signal for sequential recording on a tape or disk. During playback, recombination of the video signal is accomplished by a modulator, in place of the demodulators, and one or more delay lines for composing the modulated chrominance and luminance signals.
The main disadvantage of the line sequential recording system is the loss of vertical resolution. This loss is more serious in the areas where the broadcast system utilizes 525 horizontal lines per picture frame such as in the United States as opposed to 625 lines per frame used in Europe. Since one color component is recorded for more than one horizontal line, step function luminance transients will result in color columns because the system requires more than one line to recover and "learn" that the luminance level has changed.
A method and circuits for reducing the loss of vertical resolution in a trisequential color encoding system is known by the German Published Application DOS 2,319,820. In this known method the original color signals R, G, B are so encoded that a luminance information and three color difference signals are attained. During encoding each of the color difference signals is multiplied with a certain factor which is different from the standard factor normally associated to the same color difference signal. Subsequently the color difference signals are trisequentially switched through a low pass filter and thereafter added with the luminance signal in such a way that frequency interlace of the color spectrum lines with the luminance spectrum lines is achieved. In this system a first already expensive color video encoding process is combined with a further expensive encoding system. The high costs of the electronics do not give necessary great advantages so that the system would become applicable in an economical color video recording system.
On the other hand problems concerning the interchangeability of each trisequential system and especially relating to the different numbers of line scans for obtaining a full frame on a television screen cannot be solved without further technical solutions. In this known system the use of a large number of delay lines is a significant drawback on the way to an economical color video processing or an economical recording and/or playback system. The known system has the further great disadvantage that the full color video information is available after four or three line scan periods at the earliest.
Another method of processing the color video signal for recording and playback is disclosed in German Published Patent Application DAS 2,110,104. In this system for recording and playback of color video signals, a carrier is frequency modulated with the luminance, and a subcarrier situated below the luminance FM-frequency range is modulated with at least one chrominance signal. The frequency modulated luminance signal and the modulated chrominance signal are combined and together frequency modulated. The object of this double modulation of the luminance and chrominance signals is to prevent cross talk between the color subcarriers and the luminance signal. This method has the substantial disadvantage of requiring two modulation steps with associated error and increased cost.
The method and circuits described in the present invention use a bisequential recording system. This system is not phase sensitive and overcomes the major problems with the previously known systems as is more apparent from the discussion hereinafter.